ABSTRACT In the United States (US), the highest percentage increase in primary and secondary (P&S) syphilis rates occurred among men who have sex with men (MSM). Among MSM, the rates of P&S syphilis have increased disproportionately among young Black MSM (YBMSM). The syphilis epidemic among MSM also has been marked by a high rate of HIV co-infection for biological/virologic, behavioral, and epidemiological reasons. Syphilis and HIV epidemics overlap, in part, because syphilis facilitates HIV infection by disrupting normally protective mucosal and epithelial barriers, and this in turn causes co-infected sex partners to be at greater risk of transmitting HIV to uninfected individuals. From an epidemiological network perspective, infections with both syphilis and HIV occur most frequently in network contexts in which HIV-uninfected MSM belong to sexual networks with a high prevalence of HIV and syphilis. Such sexual networks may be formed through social spaces or venues frequented by syphilis-infected MSM, where HIV is often co-prevalent. Thus, the social venues of YBMSM may provide clues to sexual network formation and downstream syphilis infections. We conceptualize syphilis-dynamics among YBMSM as social processes that are co-evolved with HIV dynamics, sexual behavioral dynamics and network contextual dynamics (social space, sex/social/peer relationships). In addition, we conceptualize that these social processes are affected by the behavioral dynamics of ARV-based prevention factors (i.e., pre-exposure prophylaxis [PrEP] post-exposure prophylaxis [PEP], treatment as prevention [TasP], and seroadaptive/sorting behaviors. This application employs state-of-the-art longitudinal network and behavioral dynamics modeling methodologies to address the epidemiological problem of co- evolving syphilis and HIV epidemics. These methodologies have the capability to model the complex dynamic processes at multiple levels simultaneously, taking into account syphilis-HIV transmission dynamics, sex- behavioral dynamics, network dynamics, and risk-reduction behavioral dynamics among YBMSM. We propose to use existing empirical longitudinal data of YBMSM aged 16 to 29 in two large urban cities, Houston and Chicago to develop potential points of entry for intervention strategies to limit syphilis transmission (i.e., post- exposure prophylaxis, network-level syphilis treatment). Our specific aims are to: (1) model syphilis infection dynamics, co-evolved with HIV infection, sex behavior, network contexts of social space and sexual/social relationships; (2) examine how these social processes affect and are affected risk reduction behaviors and by HIV biomedical prevention. This study has the potential to deepen our understanding of the drivers of syphilis infection in the context of networks, behavior, and risk reduction. The results will lead directly to interventions that identify key levers of syphilis transmission as well as additional research that models how emerging syphilis prevention approaches such as syphilis post-exposure prophylaxis may be integrated into HIV prevention options in the U.S.